The present invention relates to a process for producing a joined ceramic component consisting of a plurality of parallel ceramic tubes and two perforated ceramic plates joined to the both ends of said tubes.
Ceramics, irrespective of their compositions (oxides or nonoxides), have high heat resistance and high heat-insulating property; electrical and electronic properties such as insulation, conductivity, magnetic and dielectric properties and the like; and excellent mechanical properties such as wear resistance and the like. Hence, ceramics usable as materials for various structures have been developed and are in actual use.
When a ceramic is used as a material for mechanical part or structural member, the mechanical part or the structural member is required to have various shapes and also it is necessary to use various parts or various structural members in combination. Therefore, it becomes necessary to integrate different ceramic parts or members into one piece when one-piece molding is difficult or impossible.
Joined components consisting of a flat-plate-like member and a member of other shape are used as mechanical parts or structural members in large amounts. There are used, in many cases, materials consisting of two perforated plates and a plurality of parallel tubes each inserted into each one hole of the two plates. In shell-and-tube type ceramic heat exchangers, for example, there are used a member consisting of a plurality of parallel ceramic tubes and two perforated plates, joined to the both ends of said tubes.
For production of such a joined ceramic component consisting of a plurality of parallel ceramic tubes and two perforated plates, bonded to the both ends of said tubes, there is known a process which comprises inserting sintered tubes into each hole of two unsintered ceramic plates 1 (as shown in FIG. 4) each having a plurality of holes 3, in such a way that the two ends of each tube are flush with the outer surfaces of the perforated plates 1, and then sintering the resulting material to join the tubes and the plates into one piece by utilizing the difference in sintering shrinkage factor between them (such a bonding process utilizing the difference in sintering shrinkage factor is hereinafter called "sintering joining").
The above sintering is conducted generally in a state that, as shown in FIG. 5, (1) a setter 4 is placed in a sagger having a sealed structure [this sealed structure is for the prevention of incoming of contaminants (e.g. carbon, etc. which are furnace materials) as well as for the control of atmosphere], (2) two perforated plates 1a and 1b are arranged, by the use of jigs 5, in parallel to each other and also to the setter 4 with a given distance provided between the two perforated plates and with the lower perforated plate 1b contacted with the setter 4, and (3) tubes 2 are inserted into the holes of the two perforated plates 1a and 1b in parallel to each other and vertically to the floor surface.
The reason why the lower perforated plate is not directly placed on the sagger and the setter is provided between the lower perforated plate and the sagger, is that when sintering is conducted in a state that the sagger and the lower perforated plate are in contact with each other, the smooth shrinkage of the lower perforated plate during sintering is prevented by the friction between the sagger and the lower perforated plate, causing deformation of the lower perforated plate. In order to prevent such deformation, sintering is conducted generally in a state that a setter made of a material having about the same sintering shrinkage factor as the perforated plates (the material is basically the same as for the perforated plates) is placed between the sagger and the lower perforated plate.
In the above process, the perforated unsintered ceramic plates having a large sintering shrinkage factor cause shrinkage during the sintering to tighten the tubes, whereby joining between the plates and the tubes is achieved. However, since the center of shrinkage of each plate is at the center of thickness of each plate, the lower plate 1b at the lower ends of the tubes rises from the setter 4 as shown in FIG. 6, during the sintering and, as a result, only the lower ends of the tubes 2 come into contact with the setter 4. Consequently, the setter 4 comes to support the total weight of the two plates 1a and 1b, the tubes 2 and the jigs 5 only at the small areas contacting with the tubes 2; the concentration of the total weight on the small contact areas destroys the setter 4; the destruction of the setter 4 incurs the cracking of the lower plate 1b.